Electrolytic polishing of metals



ELECTROLYTIC POLISHING OF METALS George H. Smith, Blasdell, N.Y.,assignor'to Electro Process, Inc., Bulfalo, N.Y.

No Drawing. Application December 16, 1950 Serial No. 201,248

8 Claims. (Cl. 204140.5)

, The present invention relates to the electro-polishing: of metals andhas particular application tothe polishing of iron, aluminum, magnesium,and copper and their alloys, such as high carbon steel, low carbonsteel, mild alloy steel, cast iron, wrought iron, stainless steel,Monel, Inconel, and brass.

forms as, for instance, plate, rods, rigidized metal, perforated metal,cold rolled and hot rolled steel and sheets and the like, and thearticles fabricated therefrom.

It is an important economic object of my invention to electro-polishmetal objects in short periods of time and at low ,anode currentdensities on the order of. v 0.25 to 10 amperes per square inch. r

It is an object of my invention to provide animprovedmethod of polishingiron, aluminum and copper and al-" loys thereof, and a composition to beemployed thereforw' It is a further object of my invention to provide a,mirror-bright finish upon the surface of the metals with simultaneousdeburring of the sharp edges.

It is a further object of the invention to provide metal pr'oductssuchas-iron and its alloys whichm'ay'be directly plated with copper, nickel,chrome or other plating metals: usually employed in the art, withoutfurther preparation.

,It is a further object of this invention to prepare a surface on suchmetals wherein there is a greater adherence of the plating metals to themetal base than has been possible heretofore ,by electro-polishingprocesses available.

It is well known to the art that a great number of methods :have beenproposed for the electro-p'olishing of steel and other metals. Ingeneral, the electrolyte was.- mainly composed of strong acids, such assulfuric acid and/or ortho-phosphoric, acid, or other acids such ashy'..

drochloric, hydrofluoric, chromic and arsenic acids, nitric acid, etc.,either used per se orv when necessary in admix-' ture in variableproportions with sulfuric acid. z 1 To these inorganic acids, additionsof a great variety-of organic compounds were made which were supposed toprovide considerable improvements over polishing soluhibitors andbuffers. I

Someof these combinations have reached widespread-7 application in theelectrolytic polishing of stainless steels. However, the application ofelectrolytic polishing to steel a'nd"ir'on has not acquired anyimportance since'the elec trolytes proposed by the art have failed tomeet the requ remsnt .for. a m n br ht fi .debnrring of the without.further preparation,

e d t en d t oni f 1 urf ce t c ul be d.

T The prior art further discloses the use of cresol sulfonicj aneffective surface brightener as illustrated in acid I The methods ofelectro-polishing 20 herein disclosed are applicable to all types ofshapes and I 2,928,777 P atented Mar. 15, 1,969

published by Chapman & Hall, London, England, 1948, but extensive testswith the use of this additive failed to"- produce a mirror-bright finishof the steel surface. In fact, it hardly showed any improvement over theuse of additives cited in the. foregoing paragraphs, and does not,

seem to have ever been adopted on an industrial scale.

I have now found, in accordance with the present invention, that a verymuch superior polishing eifect and a very smooth mirror-bright surfacecan be produced on.

. iron, aluminum, magnesium and copper, and their alloys such asstainless steel, brass, Monel, Inconel and steel, more readily and at alower cost than was heretofore possible; the employment of anelectrolytic solution for electro-polishing consisting essentially of amixture of (a) inorganic acids or a mixture of inorganic acids andaliphatic organic acids and (b) an aromatic sulfonic acid characterizedby the following general formula:

Ar(SO H) in which A-r represents an aromatic radical, single, linked orcondensed of the benzene, naphthalene, anthracene etc., series, and inwhich X is the integer 1, 2 or more.

According to my invention, lead, stainless steel or copper is used asthe cathode for the direct current (which may be produced by means of arectifier or a motor generator), the metal to be polished being theanode, both anode and cathode being substantially immersed in theelectrolytic polishing solution.

It is an important object of my invention that the current densitiesemployed are uniquely low as compared to the current densities advocatedand used by the prior art .and that I am able to polish iron and carbonsteel with current densities as low as one-half to one ampere per squareinch of anode surface, when the aromatic sulfonic acids of my inventionare used in the electrolyte of the electro-polishing bath.

The electrolyte employed, according to my invention, is composed of aplurality of acids, one of which may; be an aliphatic carboxylic acid,such as acetic or glycolicacid; the bath may include inorganic acids ormixtures thereof, such as phosphoric or sulfuric acid, or both, andanaromatic sulfonic acid of the general type described; above. I havediscovered that the addition to the electro-r polishing electrolytes ofan aromatic sulfonic acid orof a mixture of the aromatic sulfonic acidsof this inven tion is indispensible if a mirror-bright finish on steel,stainless steel, aluminum, Monel, Inconel, magnesium brass and copper,etc., is to be attained.

According to my invention I may use the aromatic sulfonic acid withphosphoric acid alone, if a bright polishv is desired on brass, copper,aluminum, carbon and stain-Q less steel, for instance; or, I may use thearomatic sulfonic acid with sulfuric acid to polish stainless steel,aluminum and carbon steel; or, I may usethe aromatic sulfonic acid withboth phosphoric and sulfuric acid to cause a bright mirror polish onaluminum, stainless steel, Monel, Inconel, magnesium, carbon steel andits alloys.

Small amounts of air-aliphatic carboxylic acid of from 1% to 20% byweight of the electrolyte may also be used in the electro-p'olishingcomposition, according to the present invention, which additions broadenthe scope of operating efficiency of such solutions over a wider rangeof temperatures. But, these additions are; not essential to theachievement of'a mirror-bright polish on any of the previously mentionedmetals and alloys ifthe aromatic sul fonic acids are being usedasillustrated by the examples given below."

' Thus metals may be polished by immersingmetalf to be treated as ananode in an electrolytic bath compris i ing 1 to 50% of an aromaticsulfonic acid of the tvpe h contemplated herein, 50 to 90% "byweight ofB6. sulfuric acid. Where an aliphatic carboxylic acid is employed in thebath the aliphatic carboxylic acid may be present from 1 to 70% byweight, the aromatic sulfonic acid from 1 to 50% by weight, with from 40to 85% by weight of 66 B. sulfuric acid. It is possible to polish metalsanodically in a bath of from S to 90% by weight of 66 B. sulfuric acidcontaining from 1 to 50% of benzene sulfonic acid.

The time of treatment may be varied over a wide range, depending uponthe acid mixture used with the aromatic sulfonic acid and the quality ofmetal being polished. Under optimal conditions, stainless steel may bepolished in one to two minutes; brass and. copper in one to two minutes;aluminum, carbon steel, Monel, Inconel, magnesium, in two to fourminutes.

The amounts of metal removed from the anode to be brightened are veryminute; the amounts removed in two minutes with an electrode potentialof 12 volts, and a typical electrolyte of the present invention is, forex- Z-naphthol 3-6-8 tri-sulfonic acid have been found especiallysuitable for imparting a bright mirror finish to carbon steel, stainlesssteel, aluminum, Monel, lnconel and magnesium.

Concentrations of the aromatic hydrocarbon sulfonic acid, or mixturesthereof, of l to by weight of the total weight of the electrolyte areeffective in improving the lustre of these metals and alloys.

Especially suitable are mixtures of naphthylaminesulfonic acids whichare by-produets and wastes from organic intermediates production, andcontain sulfuric acid in high concentration and in particular, the mono,di and tri sulfonic acid derivatives of beta-naphthylamine when used asa mixture, as they occur in the waste liquor of beta-naphthylaminesulfonation.

The following examples are given as further illustrations of the severalmethods which I have found to give the desired results, but theseexamples should not be taken as a limitation of my invention but merelyas illusample, as follows: 20 trative thereof.

(Inches E 1 Carbon steel 0.0015 xample Aluminum 0.001 24.8% by weight ofbenzene sulfonic acid of-a techni- Stainless steel (l88) 0.002 cal gradecontaining 71% free benzene mono-sulfonie Copper or brass 0.0025 acid,37.4% by weight sulfuric acid 96%,, 34.0% by Monel and Inconel 0.002weight ortho-phosphoric acid 85% and 3.8% by weight Magnesium 0.001water.

Time, iliemp. 1 Amps Metal Min. Bath Cathode Volts spa; Nature of FinishAluminum 25 pan a so. Copper 15 I 0.5 Very brightsmcoth polish.Stainless 188 17-chr0me 3 80 0--.- 10 23 Very bright polish. Low carbonsteei 5 70 Lead 10 2.0 Mirror-bright finish. High carbon steeL- 4 70 do8 1% Do. Magnesium 1% 7O Stninless-steel 8 154 Do.

The aromatic sulfonic acids which may be used, according to thisinvention, are derived from aromatic hydrocarbons such as benzene,naphthalene, anthracene, acenaphthene, phenanthrene, retene(7-isopropyl-l-methyl-phenanthrene), fluorene, etc., and may containoneor more sulfonic acid radicals, usually not more than three, to inducesolubility in the electrolyte. Aliphatic side chains may be present,such as methyl, isopropyl, butyl, ctc'., radicals. I have also foundthat, in many instances, the aromatic sulfonic acid may be substitutedby an aminogroup or a nitro group, but that the presence of a hydroxylgroup, in general, impairs the results to a considerable degree unless aplurality of sulfonic acid radicals is present.

Other substitutes, such as halogens, do not, in general, yield thedesired results because of the too great insolubility generally inherentin the halogenated aromatic sulfoacids.

Thus, the aromatic sulfonic acids which I usually prefer to use in thefree acidic state, according to my' invention, comprise the followingexamples:

Benzene mono sulfonic acid Benzene 1.3-di sulfonic acid Ortho-toluenemono sulfonic acid Para-toluene mono sulfonic aicd Meta-toluene monosulfonic acid Orthoxylene-4 sulfonic acid 2.6 naphthalene di sulfo acid2.7 naphthalene di sulfo acid Retene di sulfonic acid The aromaticsulfonic acids of the benzene series, such as bcnzene-mono-sulfonicacid, I have found to be especially effective for polishing. steel,stainless steel, aluminum and magnesium. The aromatic sulzlinnicv acidsof the naphthalene series. especially theZ-naphthylamine 5-! disulfonicacid, Z-naphthylamine 1-5-7 tri-sulfonic acid and After theelectro-polishing, the metals were rinsed in soft water or a solution ofhard water containing 1% of a softening agent, such as ethylene diaminetetra acetic acid or a sequestrating agent for the calcium and magnesiumsalts present in the hard water, and dried. This procedure was generallyfollowed in the accomplishment of allthe examples given below:

Example II Naphthalene 2,6 and 2,7 di-sulfonic acids in adequatequantities show excellent results on carbon steels, aluminum andstainless steel. in the following electrolyte: 12.4% by weight 2,6naphthalene di-sulfonic acid, 12.4% byweight 2,7 naphthalene di-sulfonicacid, 37.5% by .weiglit sulfuric. acid 96%, 34% by weightorthophosphoric acid and 3.7% by weight water.

18% by weight technical benzene sultonic acid. containing free benzenemono-sulfonic acid, 4.5% sulfuricacid (H 80 1.8%. hi-phenyl sulfone(CQHblgSQg plus unaccounted for material consisting. of water, benzoland unidentified organic byproducts, 38% by weight saga-e sulfuric acid9 6%, 40% by weight orthophosphoric acid 85% and 4% by weight water.

5% by weight retene di-sulfonic acid and 16% by weight of water. i

Time, Tem Amps. Nature of I Time Temp. Amps. Nature's! Metal Min. ofCathode Volts per Finish 5 Metal Min. of Cathode Volts per Finish" BathSq. In. Bath Sq. In.

Aluminum, 3 80 Stainless 15 0.5 Very bright, Copper 2 50 Copper.... 100.5 Mirror- 2S 36E. steel. smooth bright v finish. mb. iv 4 Stainless18-8. 3 80 ..do 10 2.3 Very bright Brass 2 50 -do 10 I 0.6 o. i 0V1pxorlish. 10 Low carbon 70 .--do l 2.0 L orsteel. brie t Aromaticsulfomc acids in the field of electrolytic High 4 70 8 1 polishingaremost efiectively used in the percentages steeld 8 1 D indicated inabove examples, i.e., from 1 to 20% by Magnesium 2 A 15 weight of thebath. 1

' However, smaller percentages may be used according- Example IV 9.3% byweight of toluene sulfonic acid 80% para 20% ortho (94%) 45.8% by weightof 96% H 80 38.5% by weight of 85% ortho-phosphoric acid, and 6.4% byweight of water.

Time, Temp. Amps. Nature oi Metal Min. of Cathode Volts per Finish BathSq. In.

Aluminum 3 35 Copper 0. 5 Smooth alloy. bright polish. Stainless 18-8. 235 Lead l0 2. 3 veryrblrllght po Low carbon 5 70 do 10 1.0 Very brightsteel. smooth polish.

1 32 s 3411, Le, 12.5% $1, 1.0 Mg, 0.9% Ni, 0.9% Cu, one-half hard.

Example V The following electrolyte gives a mirror-bright polish oncarbon steel and magnesium and consists of 4.8% by weight water, 7.8% byWeight of technical glycolic acid, 40.2% by weight of sulfuric acid 96%,38.4% by weight ortho-phosphoric acid 85%, and 8.8% by weight 71%benzene sulfonic acid. Very good results were obtained by usingpara-toluene sulfonic acid as a substitute for benzene sulfonic acid inthis formula.

50% by weight para-toluene sulfonic acid (94%), 18.5% by weight benzenesulfonic acid (71%), 31.5% by weight orthophosphoric acid (85%).

Time, Temp. Amps. Nature of Metal Min. of Cathode Volts per Finish BathSq. In.

Low carbon 5 60 Lead 10 0.8 Very bright steel. polish.

Example VII The addition of aromatic sulfonic acids to orthophosphoricacids greatly improves the lustre and brightness of polish attained oncopper and brass. Small quantities of para-nitrotoluene ortho-sulfonicacid or retene di-sulfonic acid are especially efiective in improvingthe polish on brass and copper. An example of this electrolyte is: 79%by weight of 85 ortho-phosphoric acid,

to the case. Their effect on the final polish, the current consumptionand time necessary. to achieve a mirror brightfinish, is substantial. Itis alsopossible, .by vary-: ing the percentages of the aromatic sulfonicacids-used to obtain different shades on the polished surface which mayvary from a brown tone on stainless steel, to the bluish brilliance ofchrome plated steel.

Low carbon steel polished according to Example I gives a mirror-brightfinish which can be plated with nickel or chrome without furtherpreparation.

What is claimed is:

1. A method of anodically polishing metals which comprises immersing themetal to be treated for from 1 /2 to 5 minutes as an anode in an aqueouselectrolytic bath consisting of 1.0 to 50% by weight of an aromatichydrocarbon sulfonic acid and 50 to 70% by weight of orthophosphoricacid, the current being of willcient density to overcome any etching andresult in a polishing of the metal being used as an anode.

2. A method of anodically polishing metals which comprises immersing themetal to be treated for from 1 /2 to 5 minuta as an anode in anelectrolytic bath. consisting of 1 to 50% by weight of an aromatic hydrocarbon sulfonic acid and a combination of inorganic acids of 40 to byweight said inorganic acids being selected from the group consisting ofsulfuric acid and orthophosphoric acid.

3. A method of anodically polishing metals which comprises immersing themetal to be treated for from 1 /2 to 5 minutes as an anode in anelectrolytic bath consisting of 1 to 50% by weight of an aromatichydrocarbon sulfonic acid, 1 to 7% by weight of an aliphatic carboxylicacid and 40 to 90% by weight of a combination of phosphoric and sulfuricacid.

4. A method of anodically polishing metals which comprises immersing themetal to be treated for from 1% to 5 minutes as an anode in anelectrolytic bath con sisting of 1.0 to 50% by weight of benzenesulfonic acid and 50 to 79% by weight 85% ortho-phosphoric acid, thecurrent being of suflicient density to overcome any etching and resultin a polishing of the metal being used as an anode.

5. A method of anodically polishing metals which comprises immersing themetal to be treated for from 1 /2 to 5 minutes as an anode in anelectrolytic bath con sisting of 1 to 7% of an aliphatic carboxylicacid, 1 to 50% by weight of sulfonic acid of the formula where Ar is anaromatic hydrocarbon radical and n is an integer of less than 4 theremainder being substantially inorganic acid selected from the group ofinorganic acids consisting of sulfuric acid and orthophosphoric acid.

6. A method of anodically polishing metals which comprises immersing themetal to be treated for from 1 /2 to 5 minutes as an anode in anelectrolytic bath consisting of l to 50% by weight of a sulfonic acid ofthe formula Ar(SO OH),, where Ar is an aromatic hydrocarbon radical andn is an integer less than 4, and 40 to 7 90%by weight of a combinationof phosphoric and sulfuric acids.

7. A method of anodically polishing metals which comprises immersing themetal to be treated for from IV: to 5 minutes as an anode in anelectrolytic bath consisting of 1 to 50% by weight of a sulfonic acid ofthe formula Ar(SO OH),, where Ar is an aromatic hydrocarbon radical andn is an integer less than 4,. and- 1 to 7% by weight of an aliphaticcarboxylic acid and 40 to 90% by weight of a combination of phosphoricand sul- 10 furic acids.

8; A method of anodically polishing metals which comprises immersing themetal to be treated for from 1% to 5 minutes as an anode in anelectrolytic bath consisting of 1 to 59% benzene sulfonic acid incombination with 50. to'70% of inorganic acid selected from the groupof: inorganic acids consisting of sulfuric acid and orthophosphoricacid, the time, temperature and current densities of the bath beingadjusted so as to achieve a bright polish.

8 References Cited in the file of this patent UNITED STATES PATENTS1,956,571 Grillet May 1, 1934 2,335,354 Ostrofsky Nov. 30, 19432,386,078 Weisberg et al Oct. 2, 1945 FOREIGN PATENTS 504,026 GreatBritain Apr. 17, 1939 594,932 Great Britain Nov. 21, 1947 OTHERREFERENCES Richters Organic Chemistry, vol. 2 (published in 1922), page172.

Chemical and Electroplated Finisher, by H. Silman, published in 1948' byChapman and Hall, Ltd., pages 100, 101.

1. A METHOD OF ANODICALLY POLISHING METALS WHICH COMPRISES IMMERSING THEMETAL TO BE TREATED FOR FROM 1 1/2 TO 5 MINUTES AS AN ANODE IN ANAQUEOUS ELECTROLYTIC BATH CONSISTING OF 1.0 TO 50% BY WEIGHT OF ANAROMATIC HYDROCARBON SULDONIC ACID AND 50 TO 70% BY WEIGHT OF 85%ORTHOPHOSPHORIC ACID, THE CURRENT BEING OF SUFFICIENT DENSITY TOOVERCOME ANY ETCHING AND RESULT IN A POLISHING OF THE METAL BEING USEDAS AN ANODE.